The invention relates to a device for numerical computation of a symmetrical component of an electrical quantity, current or voltage, of a three-phase power system, comprising means for measuring said quantity in at least two phases of the power system, means for sampling and for analog-to-digital conversion connected to the output of the measuring means, and means for determining said symmetrical component connected to the output of the conversion means.
It is known that an unbalanced three-phase system of sinusoidal quantities, voltages or currents, can be considered as being the superposition of three symmetrical balanced systems able respectively to be represented by the direct, inverse and zero sequence components of the quantity involved. Some protection relays, connected to a three-phase electrical power system, take account of certain of the symmetrical components of the power system voltages or currents, notably the inverse voltage or current.
In state-of-the-art devices, the values representative of the inverse component are generally computed analogically. In patent application FR-2,650,711, a shunt circuit breaker comprises a microprocessor generating the inverse voltage component from dephased voltage samples measured at a predetermined time on a first phase of the power system and respectively 13.3 ms and 6.6 ms later on the other two phases of the power system, in the case of a 50 Hz frequency power system. This method has the shortcoming of giving a wrong inverse voltage value if the phase voltage modules vary in module and in phase during the time required for computing.